1. Field of the Invention
The present invention relates to a liquid droplet recognition apparatus that detects liquid droplets attached to the front surface of a transparent member; a raindrop recognition apparatus that detects liquid droplets such as raindrops attached to a transparent member, for example, the windshield of a vehicle or the like; and an on-vehicle monitoring apparatus.
2. Description of the Related Art
For example, Patent Documents 1 through 3 disclose apparatuses that detect the state of rainfall to drive and control a wiper of a vehicle. A raindrop detection apparatus disclosed in Patent Document 1 is provided inside a vehicle, and picks up an image outside the vehicle through a windshield with an image pickup apparatus configured to have an infinite focal distance. This apparatus determines the presence or absence of rainfall in accordance with variations in the brightness level of pixels of picked-up image information from an average value.
A raindrop detection apparatus disclosed in Patent Document 2 is provided inside a vehicle, and has an image pickup apparatus and a light source that irradiates an image-pickup region of a windshield with light. This apparatus determines whether raindrops are attached to the windshield by comparing the brightness value of an image picked up by the image pickup apparatus when the light source is turned on with that of an image picked up by the image pickup apparatus when the light source is turned off.
When raindrops are detected only with the brightness information of a picked-up image as shown in Patent Documents 1 and 2, such apparatuses are susceptible to external scattering light from both the outside and inside of the vehicle while the vehicle is running, which results in lack of reliability of raindrop detection.
A raindrop sensor disclosed in Patent Document 3 detects raindrops attached to the front surface of a windshield with a stereo camera that observes a front side through the windshield. The stereo camera is composed of a stereo optical system that generates a subject image as seen from different points of view; an image pickup apparatus that generates stereo image data based on the subject image generated by the stereo optical system; and a detection unit that detects raindrops attached to the windshield based on the generated stereo image data. This camera detects the amount of raindrops according to a principle in which the scattering of light causes a change in light amount when the raindrops are attached to the windshield.
However, the raindrop sensor using the brightness information of an image picked up by this camera often malfunctions due to surrounding brightness as a disturbance. Therefore, the raindrop sensor may determine rainfall even if it is not actually raining and cause a wiper to automatically operate.
To solve this problem, a raindrop sensor disclosed in Non-Patent Document 1 is configured to arrange a LED serving as a light source and a photodiode serving as a light-receiving part such that light from the LED is totally reflected by the front surface of a windshield to be incident on the photodiode. That is, infrared light is emitted from the LED to the windshield, and light reflected by the front surface of the windshield is received at the photodiode. This raindrop sensor determines the presence or absence of raindrops based on an output of the photodiode because the light reflected by the front surface of the windshield becomes weak when the windshield gets wetted by rain. With this raindrop sensor, the light from the LED is totally reflected by the windshield when it does not rain. Therefore, an output of the photodiode is strong enough to resist a disturbance. Thus, since this raindrop sensor simply detects raindrops based on the intensity of light rather than brightness information, reliable detection of the raindrop can be ensured.
However, since this raindrop sensor must have a particular kind of lens structure in an image-pickup optical system to provide high detection sensitivity up to a detection range boundary, high manufacturing costs are inevitable. In addition, since this raindrop sensor is dedicated to detecting raindrops, it cannot be shared with other distance sensors or the like. This results in an increase in the number of sensors to be installed in a vehicle and requires a large installation space. Presently, many sensors, such as a sensor that automatically turns on and off lights, a lane deviation preventing sensor, a sensor that automatically switches high and low beams, and a sign recognition sensor, are installed in a vehicle, which in turn causes problems in reducing manufacturing costs and saving space for installing the sensors.
Patent Document 1: JP-A-2006-292543
Patent Document 2: JP-A-2005-531752
Patent Document 3: WO2004/106858
Non-Patent Document 1: Technical Review 2008 No. 20 of Mitsubishi Motors Corporation